Prepregs and laminates having a UV curable resin layer

ABSTRACT

Prepregs having a UV curable resin layer located adjacent to a first thermally curable resin layer or sandwiched between first and second thermally curable resin layers wherein the UV curable resin layer is uncured or partially cured as well as methods for preparing laminates using the prepregs wherein the laminate includes at least one UV curable resin encapsulated electrical component.

This is a divisional of U.S. patent application Ser. No. 14/838,092,filed on Aug. 27, 2015 which is a continuation of PCT/US2015/38483 filedon Jun. 30, 2015, which claims priority to U.S. provisional applicationNo. 62/019,554, filed on Jul. 1, 2014, the specifications of each ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention concerns prepregs having a UV curable resin layer and tomethods of using the prepregs to manufacture printed circuit boardhaving embedded electrical components.

(2) Description of the Art

In order to protect circuit board electronic components from damageduring the pressures and/or temperatures reached during lamination stepsin printed circuit board manufacturing processes, it is sometimesdesirable to milling a “window” in an adjacent layer to form aprotective structure in which an electronic component associated with alaminate or prepreg layer resides during lamination. A problem with thisapproach is that the prepreg layer adjacent to the electronic componentand window can be thin and provide an insufficient amount of resin tofill the window and encapsulate the electronic component during alamination step. There is a need, therefore, for new products andmethods that allow electronic components to be protected during printedcircuit board lamination steps with improved encapsulation results.

SUMMARY OF THE INVENTION

One aspect of this invention is a prepreg that may be used tomanufacture printed circuit boards or printed circuit board intermediatelaminates that have embedded electronic components where the prepregincludes a UV curable resin layer located between a first thermallycurable resin and an optional second thermally curable resin layer.

In another aspect, this invention is a prepreg including a UV curableresin layer located between a first thermally curable resin layer thatis at least partially cured and a second thermally curable resin layerthat is at least partially cured wherein the first and second thermallycurable resin layers have a thickness of from about 5 μm to about 40 μmand more preferably about 5 μm to about 15 μm and the UV curable resinlayer has a thickness of from about 10 μm to about 500 μm.

A method for embedding an electronic device in a printed circuit boardlayer comprising the steps of: applying a mask layer to a prepreg havinga first thermally curable resin layer that is at least partially cured,a second thermally curable resin layer that is at least partially curedand a UV curable resin layer positioned between the first thermallycurable resin layer and the second thermally curable resin layer whereinthe mask layer is applied to an exposed surface to one of the firstthermally curable resin layer or the second thermally curable resinlayer and wherein the mask layer has at least one UV light opaqueportion and at least one UV light transparent portion;

directing UV light at the UV mask layer for a time sufficient to curethe UV curable resin layer below the UV light transparent portion of theprepreg to form a partially UV cured prepreg including an uncured UVcurable resin portion located below the UV light opaque portion of themask layer and a cured UV curable resin portion located below the UVlight transparent portion of the mask layer;

placing a planar structured inner layer having a first and secondsurface and including at least one electrical component that is locatedon a first surface of the structured inner layer into contact with thefirst thermally curable resin layer such that the at least oneelectrical component is in registration with the at least one uncured UVcurable resin portion to form a stack; and

subjecting the stack to laminating conditions including temperatures andpressures sufficient to cause the UV light curable resin layer portionand the partially cured thermally curable resin of the first thermallycurable resin layers become liquid, flow around the electrical componentand substantially fill gaps between the prepreg and the structured innerlayer to form a laminate having at least one embedded electricalcomponent.

DESCRIPTION OF THE FIGURES

FIG. 1 is a side view of a prepreg (10) of this invention including a UVcurable resin layer (12) located between a first thermally curable(thermosetting) resin layer (14) that is at least partially cured(b-staged) and a second thermally curable resin layer (16) that is asleast partially cured;

FIG. 2 is a side view of the prepreg of FIG. 1 including a mask layer(18) having a UV light transparent portion (20) and a UV light opaqueportion (22);

FIG. 3 is a top view of the UV curable resin layer (12) of the prepregof FIG. 2 after the prepreg has been exposed to UV light wherein the UVcurable resin layer has become at least partially polymerized in thearea covered by UV light transparent portion (20) and wherein the UVcurable resin layer remains unpolymerized (uncured) in the area belowthe UV light opaque portion of mask layer (18);

FIG. 4 is a side view of the prepreg of FIG. 3;

FIG. 5 is a side view of a structured inner layer (24) including anelectrical component (26) that is soldered to a circuit trace (28)located on a first surface (30) of structured inner layer (24);

FIG. 6 is a side view of the lamination of the prepreg of FIG. 4 withthe structured inner layer (24) of FIG. 5 wherein, during the laminationprocess, the uncured UV curable resin portion and the partially curedthermally curable resin of one of the first or second thermally curableresin layers that abuts structured inner layer (24) become liquid andflow around the electrical component and fill all gaps (30) between theprepreg of FIG. 4 and the structured inner layer (24) of FIG. 5; and

FIG. 7 is a laminate that results from the lamination step shown in FIG.6.

DESCRIPTION OF CURRENT EMBODIMENTS

The present invention relates to prepregs (10) (e.g., FIG. 1) includinga UV curable resin layer (12) located between a first thermally curable(thermosetting) resin layer (14) that is at least partially cured and asecond thermally curable resin layer (16) that is at least partiallycured. The UV curable resin layer may be of any useful thickness, i.e.,thick enough to cause a sufficient amount of melted resin to flow andfill gaps created way when the electrical components associated withstructured inner layer are embedded in the UV curable resin layer duringlamination. In addition, the UV curable resin layer can have a thicknessthat is greater than the height of the electronic component that isintended to be protected by embedding it in the UV curable resin layer.The UV curable resin layer will typically have a thickness ranging fromabout 10 μm to about 500 μm, more typically from about 20 μm to about200 μm.

Any UV curable resin (or photopolymer) that is capable of becoming atleast partially liquid and flowable under normal printed circuit boardlaminating conditions may be used. Examples of useful UV curable resinsinclude, but are not limited to Ultraviolet (UV) and electron beam (EB)energy-cured materials such as urethanes acrylates, polyester acrylates,amino acrylates and epoxy acrylates. In addition, the UV curablematerials may include photoinitiators and additives that enhance theperformance of the pre and post cured materials.

The properties of a photocured material, such as flexibility, adhesion,and chemical resistance can be provided by functionalized oligomerspresent in the photocurable composite. As noted above, oligomerphotopolymers are typically epoxides, urethanes, polyethers, orpolyesters, each of which provides specific properties to the resultingmaterial. Each of these oligomers is typically functionalized by anacrylate. An example shown below is an epoxy oligomer that has beenfunctionalized by acrylic acid. Acrylated epoxies are useful as coatingson metallic substrates, and result in glossy hard coatings.

Acrylated urethane oligomers are typically abrasion resistant, tough,and flexible making ideal coatings for floors, paper, printing plates,and packaging materials. Acrylated polyethers and polyesters result invery hard solvent resistant films, however, polyethers are prone to UVdegradation and therefore are rarely used in UV curable material. Oftenformulations are composed of several types of oligomers to achieve thedesirable properties for the material.

As noted above, prepreg (10) will include an at least partially cured(b-staged) first thermally curable resin layer (14) and an optionalsecond at least partially cured thermally curable resin layer (16). Eachof these layers will typically have a thickness of from about 5 μm toabout 40 μm and more preferably from about 5 μm to about 15 μm. Thethickness of the first and second thermally curable resin layers (14,16) may be the same or different.

The first and second thermally curable resin layers may be made fromresins, resin systems or mixtures of resins that are commonly used inthe manufacture of printed circuit boards. The resin(s) will typicallybe a thermoset or thermoplastic resin. Non-limiting examples of usefulresins include epoxy resins, cyanurate resins, bismaleimide resins,polyimide resins, phenolic resins, furan resins, xylene formaldehyderesins, ketone formaldehyde resins, urea resins, melamine resins,aniline resins, alkyd resins, unsaturated polyester resins, diallylphthalate resins, triallyl cyanurate resins, triazine resins,polyurethane resins, silicone resins and any combination or mixturethereof.

In one aspect of this invention, the thermally curable resin is orincludes an epoxy resin. Some examples of useful epoxy resins includephenol type epoxy resins such as those based on the diglycidyl ether ofbisphenol A, on polyglycidyl ethers of phenol-formaldehyde novolac orcresol-formaldehyde novolac, on the triglycidyl ether oftris(phydroxyphenol) methane, or on the tetraglycidyl ether oftetraphenylethane; amine types such as those based ontetraglycidyl-methylenedianiline or on the triglycidyl ether ofpaminoglycol; cycloaliphatic types such as those based on3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate. The term“epoxy resin” also stands for reaction products of compounds containingan excess of epoxy (for instance, of the aforementioned types) andaromatic dihydroxy compounds. These compounds may behalogen-substituted. Preference is given to epoxy-resins which arederivative of bisphenol A, particularly FR-4. FR-4 is made by anadvancing reaction of an excess of bisphenol A diglycidyl ether withtetrabromobisphenol A. Mixtures of epoxy resins with bismaleimide resin,cyanate resin and/or bismaleimide triazine resin can also be applied.

The thermally curable resin compositions, in addition to a base resinwill typically include initiators or catalysts, one or more optionalflame retardants and solvents. The flame retardant may be any flameretardant material that is known to be useful in resin compositions usedto manufacture prepregs and laminates use to manufacture printed circuitboards. The flame retardant(s) may contain halogens or they may behalogen free.

Alternatively, or in addition, the thermally curable resins may includehalogens such as bromine in their backbone structure to impart the curedresin with flame retardant properties. The resin compositions may alsoinclude polymerization initiators or catalysts.

Examples of some useful initiators or catalysts include, but are notlimited to peroxide or azo-type polymerization initiators (catalysts).In general, the initiators/catalysts chosen may be any compound that isknown to be useful in resin synthesis or curing whether or not itperforms one of these functions.

The resin compositions will include one or more solvents which aretypically used to solubilize the appropriate resin compositioningredients and/or to control resin viscosity and/or in order tomaintain the resin ingredients in a suspended dispersion. Any solventknown by one of skill in the art to be useful in conjunction withthermosetting resin systems can be used. Particularly useful solventsinclude methylethylketone (MEK), toluene, dimethylformamide (DMF), ormixtures thereof.

During the manufacturing process, the reinforcing materials areimpregnated with or otherwise associated with the resin compositions andsome or most of the solvent is removed from the resin compositions toform b-staged prepregs and c-staged laminates. Thus, when resincomposition or laminate weight percent amounts are listed herein, theyare reported on a dry-solvent free-basis unless otherwise noted.

The resin compositions may include a variety of other optionalingredients including fillers, tougheners, adhesion promoters, defoamingagents, leveling agents, dyes and pigments. For example, a fluorescentdye can be added to the resin composition in a trace amount to cause alaminate prepared therefrom to fluoresce when exposed to UV light in aboard shop's optical inspection equipment. Other optional ingredientsknown by persons of skill in the art to be useful in resins that areused to manufacture printed circuit board laminates may also be includedin the resin compositions of this invention.

Referring again to FIG. 1 there is shown a side view of a prepreg (10)of this invention including a UV curable resin layer (12) locatedbetween an at least partially cured first thermally curable resin layer(14) and an at least partially cured second thermally curable resinlayer (16). In some embodiments, the thermally curable resin layers thata mask layer is not applied to during the methods described below can befully cured. The thickness of the curable resin layers should be greatenough to provide a reservoir of thermally curable resin that issufficient, alone or in combination with the UV curable resin to coverelectrical component (26) and circuit traces (28) during the laminationprocess. In some embodiments, the prepreg can include a core materialsuch as a woven glass cloth material.

FIG. 2 is a side view of the prepreg of FIG. 1 including a mask layer(18) having a UV light transparent portion (20) and a UV light opaqueportion (22). In FIG. 2, UV light (21) having a wavelength that isselected to cure the UV curable resin is directed at mask layer (18)covering or both of the first and/or second thermally curable resinlayers. The UV light passes through transparent portion (20) of mask(18) and cures the UV curable resin that underlies transparent portion(20). UV light is not able to pass through opaque portion (22) of mask(18) with the result that UV curable resin that lies below mask opaqueportion (22) remains uncured. In one embodiment of this invention notshown in the Figures, two masks that are mirror images of each other canbe applied to the surfaces of each of opposing thermally curable resinlayers (12) and (14) and the UV curing step can accomplished bydirecting UV light at both planar surfaces of the prepreg. In anotherembodiment, the two opposing masks are not mirror images of one another.

The partially UV cured prepreg resulting from the UV curing steprepresented by FIG. 2 is shown in FIGS. 3 and 4. In FIGS. 3 and 4, thepartially UV cured prepreg, includes a UV curable resin layer (12) thatincludes a least one cured UV resin portion (25) that underlies the UVlight transparent portion(s) (20) of mask layer (18) and at least oneuncured UV resin portion (27) that underlies the opaque portion(s) ofmask layer (18). The at least one uncured UV curable resin portion (27)forms the “window” in which a sensitive electronic device or componentcan be urged during later lamination steps.

FIG. 5 is a side view of a structured inner layer (24) including anelectrical component (26) that is soldered to a circuit trace (28)located on a first surface (30) of structured inner layer (24).Structured inner layer (24) may be made from components and by methodsthat are within the knowledge of one skilled in circuit boardmanufacturing. Structured inner layer (24) may be a partially or fullycured resin layer having a surface upon which circuit traces (28) areetched or vias are applied and if further includes at least oneelectrical component (28) that is attached to a circuit trace(s) and/orvia(s). Electrical component (28) may be any electrical component suchas a chip, resistor, transistor and the like that the board manufacturewishes to protect during lamination of layers in the manufacture ofprinted circuit boards. In many cases, structured inner layer willinclude a plurality of electrical components where some of theelectrical components do not need to be protected during boardlaminations. Thus, for purposes of this invention, the term “electricalcomponent(s)” refers only to those electrical components that are beingembedded in the methods described herein.

FIG. 6 is a side view showing the lamination of the partially UV curedprepreg of FIG. 4 with the structured inner layer (24) of FIG. 5. Duringthe lamination process the partially UV cured prepreg is placed over (orunder) structured inner layer (24) such that the second at leastpartially cured thermally curable resin layer (16) abuts the electricalcomponent and circuit trace features of the structured inner layer. Itshould be noted that the structure inner layer could alternatively beassociated with the first at least partially cured thermally curableresin layer. In the orientation shown in FIG. 6, the at least oneuncured UV resin portion (27) of the partially UV cured prepreg overlies(is in registration with) electrical component (26) to form a stack. Itis desirable that the at least one uncured UV resin portion (27) islarger in all dimensions than electrical component (26) that itoverlies. In certain embodiments, the partially UV cured prepreg caninclude multiple distinct uncured UV resin portions each of whichoverlie distinct electrical components attached to structured innerlayer (24).

Next, the stack is heated and pressure is optionally applied to thestack to cause the second partially cured thermal resin layer (16) andoptionally the UV hardened resin portion(s) to become flowable. Applyingpressure to the stack causes the flowable resin(s) to flow around theelectrical component and fill all gaps between the UV cured prepreg andthe structured inner layer. Maintaining the heat and pressure on thestack fully cures the partially cured thermal resin to form c-stagedresin layers (36, 38).

In the resulting laminate shown in FIG. 7, electrical component (26)becomes at least partially embedded in the uncured UV resin portion(27). This provides several potential advantages including goodencapsulation of the electrical component in the laminate and/orreducing the thickness of the resulting laminate. Good encapsulation isachieved when the at least one electrical component is protected fromdamage by the pressures and/or temperatures reached in subsequentlamination steps—if any. The resulting laminate product can be used asis or it can be used as a layer in subsequent printed circuit boardmanufacturing and lamination steps.

At this stage, the final laminate includes an uncured UV resinportion(s) (25). Uncured UV resin portion(s) (25) can remain uncured inthe final laminate. Alternatively, if the laminate is transparent or atleast partially transparent to UV light, the uncured UV resin portioncan be cured by the application of UV light to the final laminate. Bypartially transparent we mean enough UV light can pass through thematerial layer to at least partially cure the underlying UV curableresin.

Prepregs and Laminates

The thermosetting resins and UV curable resins described above areuseful for preparing prepregs shown in side-view in FIG. 1. In order tobe useful in manufacturing printed circuit boards a varnish includingone or more resins and other optional ingredients can be formulated,formed into a sheet, and partially cured—or b-staged—to form prepregsheets. As prepregs, the sheets can be laid up with additional materialsheets to form a “stack” of multiple material sheets which can befurther cured and or laminated to form a fully cured by heat and/orpressure lamination as described above.

Prepregs (10) can be manufactured in batch or in continuous processes.For example, sheets of partially cured (b-staged) thermally curableresins can be prepared and then a UV curable resin can be locatedbetween two sheets of b-staged resin to form prepreg (10). Prepregs (10)are optionally manufactured using a core material such as a roll ofwoven glass web (fabric) which is unwound into a series of drive rolls.The web then passes into a coating area where the web is passed througha tank which contains the thermosetting resin system of this invention,solvent and other components where the glass web becomes saturated withthe resin. The saturated glass web is then passed through a pair ofmetering rolls which remove excess resin from the saturated glass weband thereafter, the resin coated web travels the length of a dryingtower for a selected period of time until the solvent is evaporated fromthe web. A second and subsequent coating of resin can be applied to theweb by repeating these steps until the preparation of the prepreg iscomplete whereupon the prepreg is wound onto roll. The woven glass webcan replaced with a woven fabric material, paper, plastic sheets, felt,and/or particulate materials such as glass fiber particles orparticulate materials. In this embodiment the prepreg should includeonly one b-staged thermally curable resin layer that includes a corematerial unless the core material used is transparent to UV light of thetype used to cure the UV curable resin material. If the b-stagedthermally curable resin layer includes a UV opaque core material, thenthe mask should be applied to the opposing UV light transparent b-stagedthermally curable resin layer.

In another process for manufacturing prepregs, thermosetting resins(thermally curable resins) are premixed in a mixing vessel under ambienttemperature and pressure. The viscosity of the pre-mix can be adjustedby adding or removing solvent from the resin. The thermosetting resin(varnish) mix can be used to manufacture prepreg sheets and it can alsobe applied in a thin layer to a Cu foil substrate (RCC—resin coated Cu)using slot-die or other related coating techniques. Thus, it is possiblethat the prepregs of this invention can include on one surface apartially cured thermally curable resin layer having a copper foilsurface. If necessary some or all of the copper foil sheet can beremoved to expose the underlying UV curable resin to a UV light source.Indeed, the copper layer can be used as the mask layer and portions ofthe copper layer can be removed to form UV light transparent portions ofthe prepreg.

The term “UV curable resin” is used herein to refer to a type of resin—aresin that becomes cured upon exposure to UV light. The term is notintended to indicate the degree of cure of the resin—cured vs. uncured.

The foregoing description of the specific embodiments will reveal thegeneral nature of the disclosure so others can, by applying currentknowledge, readily modify and/or adapt for various applications suchspecific embodiments without departing from the generic concept, andtherefore such adaptations and modifications are intended to becomprehended within the meaning and range of equivalents of thedisclosed embodiments. It is to be understood that the phraseology orterminology herein is for the purpose of description and not oflimitation.

What is claimed is:
 1. A method for embedding an electronic device in aprinted circuit board layer comprising the steps of: applying a masklayer to a prepreg having a first thermally curable resin layer that isat least partially cured, a second thermally curable resin layer that isat least partially cured and a UV curable resin layer positioned betweenthe first thermally curable resin layer and the second thermally curableresin layer wherein the mask layer is applied to an exposed surface toone of the first thermally curable resin layer or the second thermallycurable resin layer and wherein the mask layer has at least one UV lightopaque portion and at least one UV light transparent portion; directingUV light at the UV mask layer for a time sufficient to cure the UVcurable resin layer below the UV light transparent portion of theprepreg to form a partially UV cured prepreg including an uncured UVcurable resin portion located below the UV light opaque portion of themask layer and a cured UV curable resin portion located below the UVlight transparent portion of the mask layer; placing a planar structuredinner layer having a first and second surface and including at least oneelectrical component that is located on a first surface of thestructured inner layer into contact with the first thermally curableresin layer such that the at least one electrical component is inregistration with the at least one uncured UV curable resin portion toform a stack; and subjecting the stack to laminating conditionsincluding temperatures and pressures sufficient to cause the UV lightcurable resin layer portion and the partially cured thermally curableresin of the first thermally curable resin layers become liquid, flowaround the electrical component and substantially fill gaps between theprepreg and the structured inner layer to form a laminate having atleast one embedded electrical component wherein the first thermallycurable resin layer does not include a cavity in which the at least oneelectrical component is located.
 2. The prepreg of claim 1 wherein atleast one of the first thermally curable resin layer and the secondthermally curable resin layer are b-staged thermally curable resinlayers.
 3. The prepreg of claim 2 wherein both the first thermallycurable resin layer and the second thermally curable resin layer areb-staged.
 4. The method of claim 1 wherein the electrical component isat least partially embedded in the UV curable resin layer.
 5. The methodof claim 1 wherein the mask layer is a copper layer that has been etchedto form the at least one UV light transparent portion.
 6. The method ofclaim 1 wherein the structured inner layer includes a plurality ofelectrical components that are to be embedded wherein each of theplurality of electrical components that are to be embedded are inregistration with a complementary uncured UV curable resin portion ofthe prepreg.
 7. The method of claim 1 wherein laminate having the atleast one embedded electrical component is an intermediate laminate thatis used in further printed circuit board manufacturing step.
 8. Themethod of claim 1 wherein the first thermally curable resin layer andthe second thermally curable resin layers each have a thickness of fromabout 5 μm to about 40 μm.
 9. The prepreg of claim 1 wherein the firstthermally curable resin layer and the second thermally curable resinlayer each have a thickness of about 5 μm to about 15 μm.
 10. Theprepreg of claim 1 wherein the UV curable resin layer has a thickness offrom about 10 μm to about 500 μm.
 11. The prepreg of claim 1 wherein theUV curable resin layer has a thickness of from about 20 μm to about 200μm.
 12. The prepreg of claim 1 wherein one of the first thermallycurable resin layer and second thermally curable resin layer is ac-staged thermally curable resin layer.
 13. The prepreg of claim 1wherein the first thermally curable resin layer includes a copper coatedsurface.